A range of environmental agents causes tissue injury that has been attributed to reactive oxygen species (ROS) produced by mitochondria. However, the causative pathways remain largely unknown because it has been difficult to directly monitor or specifically perturb ROS. In the R21 phase, this proposal brings together efforts to develop a new, genetically-targeted toolkit to perturb and measure ROS and calcium (Ca2+) signals in a sensitive and specific manner. Furthermore, this toolkit will allow recording of ROS and Ca2+ down to the level of specific subcompartments of the mitochondria, which likely make differential contributions in ROS dysregulation. The R33 phase will use adeno-associated viruses and transgene expression to bring the novel toolkit into mice to enable study of the effect of various environmental agents on ROS and Ca2+ signals in situ in the liver, heart, and skeletal muscle. Within phases one and two, the project will study the specific involvement of ROS and Ca2+ in the stress pathways triggered by arsenic (As), cadmium (Cd), and dioxin. The investigators will specifically test the novel hypothesis that environmental stress induced by these agents causes impaired mitochondria- endoplasmic/sarcoplasmic reticulum (ER/SR) functional and structural coupling, providing an important mechanism underlying cell injury in various tissues, including the liver, cardiac and skeletal muscle. This team has developed methods to manipulate the mitochondrial-ER/SR interface and has shown that this has consequences for localized Ca2+ signaling. These studies will allow for a paradigm shift in the way mitochondrial pathogenesis of environmental stress is studied, and will shed new light on the mechanisms by which environmental agents can cause tissue damage, leading to unique opportunities for the development of innovative treatment strategies.